Fig. 1

Photomicrographs of cyanobacteria and green algae in an experiment showing induced precipitation of carbonate from scattered calcite grains on sheaths to crusts enclosing filaments, with their diameters marked by red circles/dots. a Four thick young filaments of cyanobacterial Lyngbya (“L”) without induced carbonate, and a thin filament of green alga Ulothrix (“U”); b A cyanobacterial filament (Lyngbya), whose left part has scattered carbonate grains; c Enlargement of the red box in b; d A filament of Lyngbya (“L”), whose sheath is covered with carbonate grains, and four filaments of Ulothrix (“U”); e Enlargement of the red box in d; f Seven Lyngbya filaments with carbonate grains, two Lyngbya filaments free of carbonate, and many yellow-greenish filaments of Ulothrix, with a Lyngbya filament in the lower right corner completely covered by carbonate; g More than 10 cyanobacterial filaments with calcite grains on their surfaces; h Enlargement of the red box in g, the interference color is high white, indicative of calcite or aragonite. a–f: Plane-polarized light images; g, h: Cross-polarized light images